1. Field of the Invention
The present invention relates to a biodegradable molding material wherein the rate of biodegradation is adjustable and it's moldability is superior, and more particularly to a biodegradable molding material which undergoes 100% biodegradation in a short period of time, and which is used to provide short-term packaging materials as molded articles, e.g., one-way foodstuff packaging trays for display in supermarkets, and planting pots which do not require vessel recovery when seedlings or the like are transplanted.
2. Prior Art
Conventionally, foamed plastics such as foamed styrol, etc., have been mainly used in foodstuff packaging trays, which are materials for the short-term packaging of foodstuffs displayed in supermarkets. In recent years, furthermore, molded petrochemical materials such as polyvinyl chloride and polypropylene, etc., have begun to appear in planting pots in place of fired materials such as ceramics and clay, etc., as a result of the light weight and low cost of such petrochemical materials.
However, because of the permanent nature of plastic molded articles using such petrochemical materials, packaging materials such as foodstuff packaging trays, etc. after use for packaging foodstuffs and planting pots after the removal and transplantation of young plants or grown plants from the pots constitute non-biodegradable rubbish. Especially in the case of foodstuff packaging trays, disposal is a major problem, since the quantity of such materials used is large. Even in the case of disposal of such material by incineration, considerable quantities of toxic gases are generated; and the burden on incinerators is great. Accordingly, it cannot be said that such materials are easy on the environment.
In recent years, because of the problems of such plastic molded articles, various types of biodegradable resins have been developed as molding materials which can be discarded without incineration after short-term use rather than long-term use. Numerous biodegradable resins have appeared, including resins which are chemically synthesized using natural polymer type starches, chitosan, cellulose, etc. as raw materials, polyester type resins produced by microorganisms using fermentation processes, and aliphatic polyester type resins produced by chemical synthesis, etc. These resins have begun to be used in some applications as biodegradable molded articles.
The various types of biodegradable resins described above show a maximum of only 90% biodegradation even when buried in the earth for 365 days. No experimental example that shows 100% biodegradation has yet been published. Furthermore, in cases where these biodegradable resins are used "as is" in biodegradable molded articles, the thickness of the molded articles is generally 0.2 to several millimeters. Accordingly, an excessively long time is required for biodegradation. Furthermore, the moldability is poor as seen in the case of starch type resins, and the materials are expensive. Thus, these materials are not yet suitable for practical use.
Furthermore, in the case of foodstuff packaging materials such as foodstuff packaging trays, etc., which are consumed and discarded in large quantities in every household on a daily basis, there is a need for inexpensive biodegradable packaging materials which can quickly be degraded by natural processes after the contents of the packaging have been consumed. In such cases, it is conceivable that polylactic acids, which are biodegradable aliphatic polyester resins, might be used as foodstuff packaging materials. However, such resins have a low heat resistance, i.e., 59.degree. C.; accordingly, these resins cannot be steam-sterilized, and therefore, cannot be used as food containers, etc. Meanwhile, in the case of planting pots, there is a need for inexpensive molding materials which are superior in terms of moldability, which are also superior in terms of water resistance and material strength during the period of seedling growth, which quickly undergo biodegradation following transplantation of young plants transplanted into the soil together with the planting pot, and which degrade naturally without having any detrimental effect on the natural environment.